Probably Extended Attributes (http://en.m.wikipedia.org/wiki/Extended_file_attributes)

They stick better with files, as long as your tools know about them and handle them fine. Weakness is that they can (will) get lost in tools that do not. There are many of them: zip-unzip, tar-untar, http requests, ftp, etc.

> Weakness is that they can (will) get lost in tools that do not.

Including, frustratingly, Linux's NFS implementation. A WONTFIX apparently.

Ah. Also, depending on the FS, they are probably too limited, e.g. in size (only 4kb for ext2/3/4).